Integrated Geo-Mechanical and Bedding Plane Analyses to Optimize Horizontal Well Drilling Performance in Shale Oil Reservoirs

Abstract

AbstractWellbore instabilities in shales have been widely recognized to be one of the drilling performance limiters. Drilling incidences such as pack-offs, tight holes, stuck pipes, and eventual fishing operations impose great challenges to not only drilling the well safely but with upmost delivery efficiency. Due to its inherent physical characteristics of abundant geological weak layers such as bedding planes and fractures, the shale oil formation has relatively low bulk strength. Wellbore instabilities in this shale oil formation are prevalent when drilling highly deviated and/or horizontal wells. A case in point - two highly deviated wells failed to reach the well objectives because of serious wellbore instability-related problems encountered in the shale oil interval. Faced with excessive non-productive time (NPT), the oilfield operator was challenged with designing a reliable drilling plan that is based on fundamental science to drill through the problematic shale oil formation safely and effectively. The well design process began by firstly developing the geo-mechanical model of the field, particularly the in-situ stress model, which is critical for accurate wellbore stability analyses. Sensitivity studies that considered other factors including rock mechanical properties, bedding-plane characteristics, and the relationship between in-situ stress direction and beddings were then performed to assess the uncertainties in the geo-mechanical parameters on well designs.The analyses showed that the study area is currently under a normal faulting regime with the maximum horizontal principal stress direction in near parallel E-W direction. The bedding dip angle is about 10° and the dip azimuth is nearly north. Wellbore stability analyses indicated that wells drilled in a direction parallel to the shale oil bedding dip or with a small ‘attack’ angle have relatively poor stability requiring high mud weights to contain the high collapse pressure. On the other hand, wells drilled in a direction perpendicular to shale oil bedding dip or a high ‘attack’ angle would require a relatively low mud weight to maintain good wellbore stability. This finding is in constrast to the wellbore stability analysis without considering the bedding plane effects. In a particular case where the operator optimized the drilling trajectory and mud-weight requirements according to integrated geo-mechanical and bedding-plane modeling, all three planned horizontal wells were successfully drilled through the high-risk shale oil interval without any unfavorable drilling incidences except for some minor pack-offs observed during the back reaming process. The delivery of the three horizontal wells meeting well design specifications highlighted the importance of integrated geo-mechanical and bedding plane analyses in optimizing drilling performance, mitigating drilling risks, enhancing drilling safety and ultimately improving the overall well delivery efficiency when drilling through shale oil formations.